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CHAPTER 47ANIMAL DEVELOPMENT
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• The “Organizer” of Spemann and Mangold.
• Grafting the dorsal lip of one embryo onto the ventral surface ofanother embryoresults in the develop-ment of a secondnotochord and neuraltube at the siteof the graft.
• Spemann referred to the dorsal lip as a primary organizer.
Fig. 47.22
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• Preformation: the egg or sperm contains an embryo that is a preformed miniature adult.
• Epigenesis: the form of an animal emerges from a relatively formless egg.
• An organism’s development is primarily determined by the genome of the zygote and the organization of the egg cytoplasm (!!)
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• Sea Urchin - model; eggs have a jelly coat
• Acrosomal reaction -in sperm
• Cortical reaction - in egg
Fertilization activates the egg and bring together the nuclei of sperm and egg
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Acrosomal reaction: when exposed to the jelly coat the sperm’s acrosome discharges it contents by exocytosis.-Hydrolytic enzymes enable the acrosomal process to penetrate the egg’s jelly coat.-The tip of the acrosomal process adheres to the vitelline layer
•The sperm and egg plasma membranes fuse and a single sperm nucleus enter the egg’s cytoplasm.
•Na+ channels in the egg’s plasma membrane opens.
•Na+ flows into the egg and the membrane depolarizes: fast block to polyspermy.
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• The Cortical Reaction.
• Fusion of egg and sperm plasma membranes triggers a signal-transduction pathway.
• Ca2+ from the eggs ER is released into the cytosol and propagates as a wave across the fertilized egg IP3 and DAG are produced (second messengers)
• Ca2+ causes cortical granules to fuse with the plasma membrane and release their contents into the perivitelline space.
• The vitelline layer separates from the plasma membrane.
• It swells up with water
• The vitelline layer hardens into the fertilization envelope: a component of the slow block to polyspermy.
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• Activation of the Egg,
• High concentrations of Ca2+ in the egg stimulates an increase in the rates of cellular respiration and proteins synthesis.
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• In the meantime, back at the sperm nucleus...
• The sperm nucleus swells and merges with the egg nucleus diploid nucleus of the zygote.
• DNA synthesis begins and the first cell division occurs.
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• Fertilization in Mammals- similar to sea urchin
- Follicle cells - outermost covering of egg
- Zona pellucida - 2nd covering
- Whole sperm enters
Fig. 47.5
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• Cleavage follows fertilization. Zygote is POLARIZED
• Polarity is defined by the heterogeneous distribution of substances such as mRNA, proteins, and yolk.
• Yolk is most concentrated at the vegetal pole and least concentrated at the animal pole.
• In some animals, the animal pole defines the anterior end of the animal
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• In amphibians a rearrangement of the egg cytoplasm occurs at the time of fertilization.
• The plasma membraneand cortex rotatetoward the pointof sperm entry.
• The gray crescentis exposed and marksthe dorsal surfaceof the embryo.
• Cleavage occurs morerapidly in the animalpole than in thevegetal pole.
Fig. 47.7
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The zygote is partitioned into blastomeres.
• Each blastomere contains different regions of the undivided cytoplasm and thus different cytoplasmic determinants.
Cleavage partitions the zygote into many smaller cells
Fig. 47.6
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• In both sea urchins and frogs first two cleavages are vertical.
• The third division is horizontal.
• The result is an eight-celled embryo with two tiers of four cells.
Fig. 47.8a
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• Continued cleavage produces the morula.
Fig. 47.8b
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• A blastocoel forms within the morula blastula
Fig. 47.8d
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• In birds the yolk is so plentiful that it restricts cleavage to the animal pole: meroblastic cleavage.
• In animals with less yolk there is complete division of the egg: holoblastic cleavage.
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Gastrulation rearranges the embryo into a triploblastic gastrula with a primitive gut.
• The embryonic germ layers are the ectoderm, mesoderm, and endoderm.
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• Sea urchin gastrulation.
• Begins at the vegetal pole where individual cells enter the blastocoel as mesenchyme cells.
• The remaining cells flatten and buckle inwards: invagination.
• Cells rearrange to form the archenteron.
• The open end, the blastopore, will become the anus.
• An opening at the other end of the archenteron will form the mouth of the digestive tube.
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• Frog gastrulation
-Where the gray crescent was located, invagination forms the dorsal lip of the blastopore.
-Cells on the dorsal surface roll over the edge of the dorsal lip and into the interior of the embryo: involution.
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• The derivatives of the ectoderm germ layer are:
• Epidermis of skin, and its derivatives
• Epithelial lining of the mouth and rectum.
• Cornea and lens of the eyes.
• The nervous system; adrenal medulla; tooth enamel; epithelium of the pineal and pituitary glands.
In organogenesis, the organs of the animal body form from the three embryonic germ layers
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• The endoderm germ layer contributes to:
• The epithelial lining of the digestive tract (except the mouth and rectum).
• The epithelial lining of the respiratory system.
• The pancreas; thyroid; parathyroids; thymus; the lining of the urethra, urinary bladder, and reproductive systems.
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• Derivatives of the mesoderm germ layer are:
• The notochord.
• The skeletal and muscular systems.
• The circulatory and lymphatic systems.
• The excretory system.
• The reproductive system (except germ cells).
• And the dermis of skin; lining of the body cavity; and adrenal cortex.
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• The amniote embryo is the solution to reproduction in a dry environment.
• Shelled eggs of reptiles and birds.
• Uterus of placental mammals.
Amniote embryos develop in a fluid-filled sac within a shell or uterus
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• Avian Development.
• Cleavage is meroblastic, or incomplete.
• Cell division is restricted to a small cap of cytoplasm at the animal pole.
• Produces a blastodisc, which becomes arranged into the epiblast andhypoblast thatbound theblastocoel, theavian versionof a blastula.
Fig, 47.12 (1)
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• During gastrulation some cells of the epiblast migrate (arrows) towards the interior of the embryo through the primitive streak.
• Some of these cells move laterally to form the mesoderm, while others move downward to form the endoderm.
Fig, 47.12 (2)
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• In early organogenesis the archentreron is formed as lateral folds pinch the embryo away from the yolk.
• The yolk stalk (formed mostly by hypoblast cells) will keep the embryo attached to the yolk.
• The notochord, neural tube, and somites form as they do in frogs.
• The three germlayers and hypoblastcells contribute tothe extraembryonicmembrane system.
Fig, 47.12 (3)
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• The four extraembryonic membranes are the yolk sac, amnion, chorion, and allantois.
• Cells of the yolk sac digest yolk providing nutrients to the embryo.
• The amnion encloses the embryo in a fluid-filled amniotic sac which protects the embryo from drying out.
• The chorion cushions the embryo against mechanical shocks.
• The allantois functions as a disposal sac for uric acid.
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Mammalian Development
• Cleavage is slower• A blastocyst includes the blastocoel and the trophoblast• The trophoblast forms the fetalportion of the placenta• The blastocyst implants in the uterine lining• The 4 extraembryonicmembranes are the chorion,amnion, allantois, and yolk sac
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• Mammalian Development.
• Recall:
• The egg and zygote do not exhibit any obvious polarity.
• Holoblastic cleavage occurs in the zygote.
• Gastrulation and organogenesis follows a pattern similar to that seen in birds and reptiles.
• Relatively slow cleavage produces equal sized blastomeres.
• Compaction occurs at the eight-cell stage.
• The result is cells that tightly adhere to one another.
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• Step 1: about 7 days after fertilization.
• The blastocyst reaches the uterus.
• The inner cell mass is surrounded by the trophoblast.
Fig. 47.15 (1)
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• Step 2: The trophoblast secretes enzymes that facilitate implantation of the blastocyst.
• The trophoblast thickens, projecting into the surrounding endometrium; the inner cell mass forms the epiblast and hypoblast.
• The embryo will develop almostentirely from the epiblast.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 47.15 (2) and (3)
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Fig. 47.15 (2) and (3)
• Step 3: Extraembryonic membranes develop.
• The trophoblast gives rise to the chorion, which continues to expand into the endometrium and the epiblast begins to formthe amnion.
• Mesodermal cells are derived from the epiblast.
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Step 4:
Gastrulation: inward movement of epiblast cells through a primitive streak form mesoderm and endoderm.
Fig. 47.15 (4)
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Once again, the embryonic membranes – homologous with those of shelled eggs. Chorion: completely surrounds the embryo and other
embryonic membranes. Amnion: encloses the embryo in a fluid-filled amniotic
cavity. Yolk sac: found below the developing embryo.
Develops from the hypoblast. Site of early formation of blood cells which later
migrate to the embryo. Allantois: develops as an outpocketing of the embryo’s
rudimentary gut. Incorporated into the umbilical cord, where it forms
blood vessels.
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• Changes in cellshape usuallyinvolvesreorganizationof thecytoskeleton.
1. Morphogenesis in animals involves specific changes in cell shape, position, and adhesion
Fig. 47.16
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• The cytoskeleton is also involved in cell movement.. Cell crawling is involved in convergent extension.
• The movements of convergent extension probably involves the extracellular matrix (ECM).
• ECM fibers may direct cell movement.
• Some ECM substances, such a fibronectins, help cells move by providing anchorage for crawling.
• Other ECM substances may inhibit movement in certain directions.
• Cell adhesion molecules (CAMs): located on cell surfaces bind to CAMs on other cells.
• Differences in CAMs regulate morphogenetic movement and tissue binding.
Fig. 47.17
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• In many animal species (mammals may be a major exception), the heterogeneous distribution of cytoplasmic determinants in the unfertilized egg leads to regional differences in the early embryo
• See Chapter 21
2. The developmental fate of cells depends on cytoplasmic determinants and cell-cell induction: a review
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• Subsequently, in induction, interactions among the embryonic cells themselves induce changes in gene expression.
• These interactions eventually bring about the differentiation of the many specialized cell types making up a new animal.
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• Fate maps illustrate the developmental history of cells.
• “Founder cells” give rise to specific tissues in older embryos.
• As development proceeds a cell’s developmental potential becomes restricted.
3. Fate mapping can reveal cell genealogies in chordate embryos
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 47.20
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• Polarity and the Basic Body Plan.
• In mammals, polarity may be established by the entry of the sperm into the egg.
• In frogs, the animal and vegetal pole determine the anterior-posterior body axis.
4. The eggs of most vertebrates have cytoplasmic determinants that help establish the body axes and differences among cells of the early embryo
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• Restriction of Cellular Potency.
• The fate of embryoniccells is affected byboth the distributionof cytoplasmicdeterminants andby cleavage pattern.
Fig. 47.21
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• Induction: the influence of one set of cells on a neighboring group of cells.
• Functions by affecting gene expression.
• Results in the differentiation of cells into a specific type of tissue.
5. Inductive signals drive differentiation and pattern formation invertebrates
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An example of the molecular basis of induction:
Bone morphogenetic protein 4 (BMP-4) is a growth factor.
• In amphibians, organizer cells inactivate BMP-4 on the dorsal side of the embryo.
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• Pattern Formation in the Vertebrate Limb.
• Induction plays a major role in pattern formation.
• Positional information, supplied by molecular cues, tells a cell where it is relative to the animals body axes.
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• Limb development in chicks as a model of pattern formation.
• Wings and legs begin as limb buds.
• Each component of the limb is oriented with regard tothree axes:
• Proximal-distal
• Anterior-posterior
• Dorsal-ventra.
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Fig. 47.23b
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Organizer regions.
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Fig. 47.23a
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• Apical ectodermal ridge (AER).
• Secretes fibroblast growth factor (FGF) proteins.
• Required for limb growth and patterning along the proximal-distal axis.
• Required forpattern formationalong thedorsal-ventralaxis.
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Fig. 47.23a
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• Zone of polarizing activity (ZPA).
• Secretes Sonic hedgehog, a protein growth factor.
• Required for pattern formation of the limb along the anterior-posterior axis.
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• Homeobox-containing (Hox) genes play a role in specifying the identity of regions of the limb, as well as the body as a whole.
• In summary, pattern formation is a chain of events involving cell signaling and differentiation.
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